Radioligand binding studies of the Alpha2-adrenergic receptors that modulate platelet aggregation are an important method for studying regulation and possible abnormalities of adrenergic receptors in people. However, clinical studies are hampered by our hazy understanding of how these receptors are coupled to platelet aggregation. In order to increase our understanding of the platelet Alpha2-adrenergic receptors, I propose several novel experiments. (1) It is clear that receptors dynamically attach to and detach from coupling proteins that, in turn, interact with adenylate cyclase. Studying the relationships between receptors and coupling proteins with conventional radioligand binding experiments is difficult because the experimental conditions can change that relationship. I propose to overcome this problem by using platelets fixed with cross linking agents. (2) Na+ influences agonist binding to receptors in membranes prepared from platelets but the physiological significance of this is unclear. I propose several experiments with intact platelets to find out whether Na+ acts from inside the platelet. I then plan to determine whether physiological or pathological alterations in this internal sodium concentration can alter platelet function. (3) At physiological concentrations, epinephrine does not initiate platelet aggregation but does potentiate aggregation due to their agents, such as ADP, whose effects are mediated by other types of receptors. The mechanism for this "pro-aggregation" response to epinephrine is totally unknown. I plan to test the hypothesis that the two types of receptors are synergistic because they both interact with a common pool of coupling proteins. (4) Aggregation initiated by epinephrine is blunted in platelets previously incubated with extremely high concentrations of epinephrine. Incubations with physiologically concentrations of epinephrine, however, do not have this effect and I plan to determine whethe the potentiation of ADP-induced aggregation is desensitized and, if so, whether alterations in radioligand can be detected. These experiments should ultimately lead to improved methods for assessing the Alpha2-adrenergic receptors on intact platelets in clinical studies.